Conventional electric mirrors for vehicles mainly use an ECM (electrochromic material) technology. This technology was made well known through U.S. Pat. No. 4,902,108, U.S. Pat. No. 5,204,778, U.S. Pat. No. 5,278,693, U.S. Pat. No. 5,280,380, U.S. Pat. No. 5,282,077, U.S. Pat. No. 5,448,397, U.S. Pat. No. 5,451,822, U.S. Pat. No. 6,512,624, etc. This technology has been commercialized at present, and products using it have been applied to several million vehicles.
Furthermore, an electric mirror for vehicles using an LCD was proposed in U.S. Pat. No. 4,676,601. The critical points of this patent are limited to a light sensing technique and a technique of determining whether to operate the LCD.
To date, most commercialized electric mirrors for vehicles are products using the ECM technology, but electric mirrors using LCDs are not yet commercialized. However, conventional ECM rear view mirrors, which have been produced, have a slow response speed in a range from 3 to 6 seconds and relatively high power consumption of several hundreds of mA. In the case of LCD mirrors, because a polarizing sheet is used, reduction in light reflectance due to light absorption always occurs. Therefore, because the mirror is slightly dark even when not in operation, there is difficulty applying it to a mirror for vehicles.
Generally, a polarization rate P(%) of a polarizing sheet is explained by the equation P=100×(T1−T2)/(T1+T2). In this equation, T1 denotes light transmissivity of the polarization sheet when the vibration direction of incident light is parallel with the polarization axis of the polarization sheet, and T2 denotes light transmissivity of the polarization sheet when the vibration direction of incident light is perpendicular to the polarization axis of the polarization sheet.
Typically, polarizing sheets used in TFT (thin film transistor) LCDs or STN (super twisted nematic) LDCs have a polarization rate of 95% or more in the visible light band and light transmissivity in the range from 38% to 45%. In the case of the TFT LCDs or STN LCDs manufactured using such polarizing sheets, a contrast ratio, which is the ratio of the luminance between the brightest state and the darkest state, ranges from 20:1 to 1000:1. In the case of an LCD mirror manufactured by coupling such an LCD panel to a reflective plate, due to the difference in luminance between the brightest state and darkest state of a polarizing sheet, when in a dark state, the mirror becomes excessively dark, and, even when in a bright state, the mirror is relatively dark, having light reflectance of approximately 25%. Hence, this LCD mirror is not suitable for rear view mirrors for vehicles.
In the case of a ‘Guest-Host LCD Mode’, which uses no polarizing sheet or only one polarizing sheet, the brightness thereof varies greatly depending on the viewing angle. Furthermore, even when in operation, because the light cut rate thereof is only approximately 8%, it is insufficient to prevent the driver from being dazzled. Furthermore, because the dyeing material used as the ‘Guest’ is not reliable with respect to ultraviolet rays, it is not suitable for rear view mirrors for vehicles.